1. Field of the Invention
The present invention relates to an apparatus for developing an electrostatic latent image which has been generated on a recording medium. More particularly, the present invention relates to an apparatus employing an electrically charged single component developer which is conveyed, by a developing roller, onto the surface of the recording medium.
2. Background and Prior Art
An apparatus of the above type is known from U.S. Pat. No. 4,286,543 to Ohnuma et al which describes an apparatus in which a device for applying a developer is brought into pressure contact with a recording medium at a defined pressure. The device has an endless, conducting surface which yields resiliently, and the latent charge image on the recording medium is developed by a single component developer. The device for applying the developer and the recording medium are moved in the same direction. The peripheral speed of the device for applying the developer is somewhat greater than that of the recording medium. The apparatus employed for the process possesses a blade electrode, which bears against the surface of the device at a defined pressure for applying the developer, and which charges the single component developer with a predetermined polarity by charge injection.
The ratio of the peripheral speed of the device for applying the developer to that of the recording medium lies within the range from 1.1:1 to 1.5:1.
The most commonly used developing processes employ two-component developers, which are composed of toner and carrier particles. Recently, single component developers have been employed. These developers generally involve magnetic single component toners. When a two-component developer is used, it is necessary to measure the toner consumption continously inasmuch as the toner is constantly being carried out by the copies, and to effect a metered replenishment of the toner in a manner which ensures that the ratio of toner to carrier particles remains substantially constant. Since, in the case of a two-component developer, the carrier particles are not consumed, their long residence time in the developing system of the copier causes them to be subject to an aging process, for example as a result of corrosion, which leads in turn to precipitation of the toner by the carrier particles. This results in undesirably large quantities of toner precipitating onto the background of the copy. In the case of the two-component developers, it is also disadvantageous to have a large amount of developer continuously present in the developing system inasmuch as long residence times of the toner in the developer will result in the quantity of the former being reduced by comminution processes and the process whereby the aging of the developer as a whole is accelerated.
Magnetic single component developers do not exhibit the above-mentioned disadvantages but require expensive and sometimes complicated magnetic components, such as for example magnetic brushes, in order to transport them. Even in the case of magnetic single component developers, a certain difficulty results from the fact that, due to their magnetic constituents, they possess a higher electrical conductivity. As a result, the process of charging the magnetic toner, by tribo-electricity, is rendered more difficult.
A developing apparatus for a single component developer is known from German Offenlegungsschrift No. 3,107,055. That document describes an apparatus in which a thin, uniform layer of an insulating, non-magnetic, single component developer is formed on a developer carrier, and a latent image carrier is positioned opposite the developer carrier in order to develop a latent image which has been generated on the latent image carrier. A coating device is provided for applying the developer to the developer carrier, in a defined layer thickness. The coating device may comprise, for example, a grid and a pressure element for pressing the grid against the developer carrier.
A developing apparatus for a two-component developer is known from German Offenlegungsschrift No. 2,345,827. The apparatus described in this reference comprises a magnetic brush with a roller brush bearing against its periphery. The roller brush scatters the developer from the surface of the magnetic brush, and wires are provided for removing the developer from the bristles of the roller brush. The cloud of developer created as a result of this scattering process moves towards a photoconductive drum carrying the latent image which is to be developed. A wire mesh is located at a short distance from the surface of the photoconductive drum. The wire mesh is configured as a counter electrode to the photoconductive surface, and is intended to prevent insufficiently charged toner particles from reaching the recording medium. However, the particles which are held back cause meshes of this type to become clogged.
In devices for applying single component developer which comprise blade electrodes for limiting the layer thickness, or which comprise a pressure element, with a grid, bearing against a developing roller, the developer can undergo an agglomeration and compaction process, which takes place under pressure. As a result, the desired formation of a uniformly thick and uniformly charged layer of toner may be prevented.